1. Field of the Invention
This invention relates to a radiation image recording method and apparatus. This invention particularly relates to a radiation image recording method and apparatus for use in a cone-beam computed tomography system (a cone-beam CT system).
2. Description of the Prior Art
Techniques for recording and reproducing radiation images have widely been used in practice, wherein a radiation image having been obtained by irradiating radiation to an object and causing the radiation carrying image information of the object to impinge upon a radiation detector is converted into an image signal, the image signal is subjected to appropriate image processing, and a visible image is reproduced from the processed image signal. FIG. 6 is a perspective view showing a conventional X-ray tomographic image recording apparatus 100, in which X-rays 108 are irradiated in a cone beam-like form to an object 102. The X-ray tomographic image recording apparatus 100 is disclosed in, for example, Japanese Unexamined Patent Publication No. 9(1997)-253079. The X-ray tomographic image recording apparatus 100 comprises an X-ray source 104 and a two-dimensional detector 106, which are located so as to stand facing each other with the object 102 intervening therebetween. The X-ray source 104 and the two-dimensional detector 106 are rotated in a pair around the object 102. During the rotation, at a plurality of positions of rotation, the X-rays 108 carrying image information of the object 102 are detected as X-ray images by the two-dimensional detector 106. X-ray image signals representing the X-ray images having thus been obtained are fed into an image processing section 110 and utilized for reconstructing a three-dimensional image. In this manner, a tomographic image of a desired cross-section of the object can be obtained.
However, with the cone beam-like X-rays 108, the range irradiated simultaneously to the X-rays 108 is wide. Therefore, when the cone beam-like X-rays 108 passes through the object 102, much scattered X-rays occur. The problems thus occur in that the scattered X-rays impinge upon the two-dimensional detector 106, and much noise occurs in the detected image. In order to solve the problems with regard to noise due to the scattered X-rays, a technique has heretofore been utilized, wherein a scattered radiation removing grid is located between the object 102 and the two-dimensional detector 106 in order to remove the scattered X-rays occurring when the X-rays 108 pass through the object 102.
However, in cases where the scattered radiation removing grid is located between the object 102 and the two-dimensional detector 106, even if the scattered X-rays are removed by the grid, the problems occur in that the grid causes moire to occur in the image during the image processing. Further, part of the X-rays 108 carrying the image information of the object 102 is absorbed by the grid, and a signal-to-noise ratio of the detected image signal becomes low.
The primary object of the present invention is to provide a radiation image recording method, wherein scattered radiation is capable of being removed efficiently such that no loss of an image signal due to a grid occurs.
Another object of the present invention is to provide an apparatus for carrying out the radiation image recording method.
The present invention provides a radiation image recording method, comprising the steps of:
i) locating a radiation source and a radiation detector with an object intervening therebetween,
ii) locating a set of radiation blocking plates, each of which has at least one slit, between the radiation source and the radiation detector such that the object intervenes between the radiation blocking plates, and such that the slits of the radiation blocking plates are aligned in a straight line with the radiation source,
iii) shifting the set of the radiation blocking plates stepwise in a direction along which radiation having been produced by the radiation source scans the object, such that the state in which the slits of the radiation blocking plates and the radiation source are aligned with one another in the straight line is kept,
iv) after each step of the shifting, rotating the radiation source, the set of the radiation blocking plates, and the radiation detector around the object, and
v) recording radiation image patterns of the object during the rotation.
The term xe2x80x9cradiation scanning an objectxe2x80x9d as used herein means that the radiation, which has passed through the slit of the radiation blocking plate close to the radiation source and has been shaped by the slit into a fan beam, moves with respect to the object and in the direction which intersects with the plane of the fan beam.
The present invention also provides a radiation image recording apparatus, comprising:
i) a radiation source,
ii) a radiation detector, which is located so as to stand facing the radiation source with an object intervening between the radiation detector and the radiation source,
iii) a set of radiation blocking plates, each of which has at least one slit and which are located between the radiation source and the radiation detector such that the object intervenes between the radiation blocking plates, and such that the slits of the radiation blocking plates are aligned in a straight line with the radiation source,
iv) means for shifting the set of the radiation blocking plates stepwise in a direction along which radiation having been produced by the radiation source and having been shaped by the slit of the radiation blocking plate close to the radiation source into a fan beam scans the object, such that the state in which the slits of the radiation blocking plates and the radiation source are aligned with one another in the straight line is kept,
v) means for rotating the radiation source, the set of the radiation blocking plates, and the radiation detector around the object after each step of the shifting, and
vi) image recording means for recording radiation image patterns of the object at a plurality of positions of rotation during the rotation.
With the radiation image recording method and apparatus in accordance with the present invention, the set of the radiation blocking plates, each of which has at least one slit, are located with the object intervening therebetween. The set of the radiation blocking plates are shifted stepwise in the direction along which the radiation having been produced by the radiation source scans the object, such that the state in which the slits of the radiation blocking plates and the radiation source are aligned with one another in the straight line is kept. Also, after each step of the shifting, the radiation source, the set of the radiation blocking plates, and the radiation detector are rotated around the object. During the rotation, radiation image patterns of the object are recorded. Therefore, the radiation image recording method and apparatus in accordance with the present invention have the effects described below. Specifically, the radiation having been produced by the radiation source is restricted by the slit of the radiation blocking plate, which is close to the radiation source, into a fan beam-like shape. The fan beam impinges upon the object. Therefore, little scattered radiation occurs when the fan beam passes through the object. Also, the scattered radiation coming from the object is removed by the slit of the radiation blocking plate, which is close to the radiation detector. As a result, image patterns containing little noise can be obtained. Further, the set of the radiation blocking plates are shifted stepwise, the entire image recording system is rotated around the object after each step of the shifting, and the radiation image patterns of the object are recorded during the rotation. In this manner, the image patterns of the entire area of the object are obtained. From the thus obtained image patterns, a three-dimensional image having good image quality can be reconstructed. In cases where each of the radiation blocking plates has a plurality of slits, and a plurality of fan beams are formed, a large amount of signal can be obtained from a single recording operation. Therefore, in such cases, the image recording operations can be performed quickly, and the image recording efficiency can be kept high.